The present invention is directed to a method of and apparatus for forming a can end which is highly resistant to internal pressure when seamed to a product-containing can.
Typical of one conventional method of manufacturing so-called pressure resistant can ends is that disclosed in U.S. Pat. No. 4,109,599 in the name of Freddy R. Schultz issued Aug. 29, 1978 and assigned to Aluminum Company of America. In accordance with one method disclosed in this patent, a sheet metal blank is positioned between a pair of dies which are moved to first shear an edge of the blank after which a punch descends to form the now circular blank about an annular ring into an end shell having a peripheral flange, a frusto-conical wall, a radius and an end panel. The end shell is then removed from the first set of dies and inserted into a second set of dies in which the peripheral flange is curled into a downward peripheral flange suitable for double seaming operations.
The end shell is than placed between another pair of dies which when moved toward each other form the radius into a reinforcing channel or annular groove adjoining the simultaneously formed domed central panel. The so-called reinforcing channel or annular groove increases the pressure resistance of the can end because of the reinforcement created by the increased depth of the annular groove with respect to the central panel and the tight radius of curvature of the latter. This type of reinforcement is said to make it possible to reduce the gauge thickness of a can end about 10 to 20 percent while maintaining pressure resistance capabilities of a conventional can end. However, the patent also acknowledges two dichotomous principles which are at work in the manufacture of a pressure resistance can end of this type, namely, the deepening of the annular groove and the tightening of its radius acts to increase pressure resistance, but the drawing operation has the effect of thinning the metal which acts to decrease pressure resistance.
While the objectives of conventional methods and apparatus are acknowledged herein, it is also important to recognize that such known methods also include other disadvantages, particularly when a blank or end shell must be transferred between a first set of dies to a second set of dies which virtually necessarily create alignment and/or tolerance problems, not to mention the simple fact that the transfer itself adds time to an overall forming operation simply because of the time involved in the transfer per se. Furthermore, it is not uncommon to lacquer the blanks prior to any forming operation, and forming in different dies and/or transferring between dies increases the tendency of the lacquer or enamel to crack or otherwise expose the metal to the eventual product packaged within a can to which the end has been seamed. The latter can result in undesired product deterioration.
Another disadvantage of forming a pressure-resistant can end in a series of different dies between which the blank must be transferred is simply the inability to maintain acceptable tolerances, particularly relative to overall concentricity, flange height and hook length. These three factors collectively establish to a large measure the eventual uniformity of successful double seaming which, once again, can be critical to product shelf life and/or longevity.